The present invention relates to diamond cutting elements for use in rotary drill bits which cut bores in subterranean formations.
In a typical rotary drilling operation, a rotary drill bit is rotated while being advanced into a soil or rock formation. The soil or rock is cut by cutting elements on the drill bit, and these cuttings are flushed from the borehole by the circulation of drilling fluid toward the top of the borehole. The drilling fluid is delivered to the drill bit downwardly through a passage in the drill stem and is ejected outwardly through nozzles disposed in bores in the cutting face of the drill bit. The ejected drilling fluid is directed outwardly through the nozzles at high speed to aid in cutting, and to flush the cuttings and cool the cutter elements.
Conventional cutting elements comprise a stud having an inclined supporting surface at its front end, and a cutting disk mounted on the supporting surface. The disk comprises a substrate having one surface braze-bonded to the supporting surface and a second surface which carries a diamond substance such as a layer of polycrystalline diamond or thermally stable diamond. The stud and substrate are normally formed of a hard material such as cemented carbide.
As depicted in FIGS. 6-9, it has heretofore been proposed to promote the adherence of the substrate to the stud by providing an arrangement of intermeshing teeth, i.e., V-shaped projections (1) in the substrate (2) and supporting surface (3) of the stud (4). The teeth extend completely across the substrate and supporting surface in a side-to-side direction, i.e., wherein the teeth lie in planes (5) intersecting the longitudinal axis of the stud. A brazing alloy (6) is sandwiched between the teeth of the substrate and supporting surface. In certain respects, the provision of such intermeshing teeth may be considered as improving the connection between the substrate and stud. However, in other respects, such intermeshing teeth create certain disadvantages. For example, the present inventor has observed that in some instances the premature breakage of cutting elements stems from cracks on the stud which initiate from an edge (7) of the stud where the supporting surface intersects the cylindrical side surface. In that regard, cracks have been found to initiate at the irregularities created in the edge (7) by the teeth. In particular, cracks were observed to initiate along that region (8) of the edge situated longitudinally rearwardly of a longitudinal front half (9) of the substrate, as those front and rear directions are considered with reference to the direction of longitudinal advancement of the drill bit. There results, therefore, a tendency for cracks to propagate from that region which weaken the stud and lead to premature breakage thereof.
Consequently, the use of intermeshing projections in the stud and supporting surface has not achieved commercial success.